Move all APR functions related to strings to their own directory, and

create a new header for those functions.  This is the first step to
removing the apr/lib directory completely, and moving those files/functions
to descriptive directories.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85867 13f79535-47bb-0310-9956-ffa450edef68